The present invention generally relates to the formation of a nickel metal hydride (NiMH) electrochemical cell. More particularly, the present invention relates to the formation of a cobalt (III) conductive matrix on and around particles of electrochemically active nickel compound in a positive electrode of a NiMH electrochemical cell.
NiMH cells, particularly those NiMH cells having foam positive electrodes, are well known for their high capacities and energy densities. In typical NiMH cells, the negative electrode is formed of a metal hydride while the positive electrode is formed of nickel hydroxide Ni(OH).sub.2. The NiMH cells are assembled in a discharged state. During charging, the Ni(OH).sub.2 converts to nickel oxyhydroxide (NiOOH), which converts back to Ni(OH).sub.2 upon discharge. Because Ni(OH).sub.2 is not electrically conductive, an electrically conductive matrix must be formed around the Ni(OH).sub.2 particles. A common conductive matrix used in such positive electrodes is made of cobalt oxyhydroxide (CoOOH). To form a conductive matrix of CoOOH around Ni(OH).sub.2 particles, spherical Ni(OH).sub.2 particles and oxides of cobalt are physically mixed to form a paste. One cobalt oxide compound that may be included in the paste is CoO. The cobalt oxide compound dissolves in an alkaline electrolyte to form HCoO.sub.2.sup.- ions, which surround the spherical Ni(OH).sub.2 particles and re-precipitate as cobalt hydroxide (Co(OH).sub.2). The conductive network of CoOOH is formed when the electrode is charged in accordance with a formation procedure. Relatively low charging rates are used for long periods of time to form the conductive network and to activate the Ni(OH).sub.2 to NiOOH. This can be time consuming and labor intensive since this formation procedure involves charging an assembled cell with a constant current of C/20 for thirty-two hours.
One function of the formation procedure is to convert all the Co(OH).sub.2, which includes cobalt in its +2 valance state (i.e., Co(II)), to CoOOH, which includes cobalt in its +3 valance state (i.e., Co(III)). If the cobalt is not converted completely to CoOOH, other species of cobalt are formed such as Co.sub.3 O.sub.4, which is electrochemically inactive.
Also, the cobalt that is left behind in the +2 valance state dissolves in the alkaline electrolyte as HCoO.sub.2.sup.- ions. The HCoO.sub.2.sup.- ions are very mobile, and move to and through the separator and plate the separator or the metal hydride negative electrode. The unconverted HCoO.sub.2.sup.- ions readily get converted to Co.sub.3 O.sub.4 in the presence of oxygen, which is obtained when the positive electrode self-discharges to form Ni(OH).sub.2. The migration and conversion of the HCoO.sub.2.sup.- ions cause soft shorts and a redistribution of cobalt in the negative electrode. Because Co.sub.3 O.sub.4 is electrochemically inert and acts as an insulator within the electrode, the formation of Co.sub.3 O.sub.4 reduces the charge and discharge efficiency of the positive electrode. Longer soak times (i.e., the time during which the electrode is soaked in electrolyte so as to dissolve the CoO in high temperatures) accelerate this process.
Since part of the Co(II) is converted to Co.sub.3 O.sub.4, less Co(II) is available to form the Co(III) conductive matrix during the formation procedure. Due to the detrimental effects of having Co.sub.3 O.sub.4 form in the positive electrode, a method is needed for forming a NiMH cell that maximizes the percentage of Co(II) converted and maintained as Co(III).
Another function of the formation procedure is to activate as much of the Ni(OH).sub.2 as possible in the positive electrode. In many cases the Ni(OH).sub.2 at the outer surface of the electrode is charged to form NiOOH. This is followed by oxygen evolution. Once the oxygen evolution begins, the electrode no longer accepts charge to form the remaining Ni(OH).sub.2, which may be buried deep in the electrode. Activating as much of the Ni(OH).sub.2 as possible not only influences the first cycle capacity, but also affects the performance of the cells during consecutive cycles. Therefore, there further exists the need for a formation procedure that maximizes the amount of Ni(OH).sub.2 that is activated.
Another practical problem relating to the manufacture of NiMH cells results from storage of such cells for long periods of time after formation. When NiMH cells are stored for periods of up to nine months, they exhibit significant losses in capacity on the order of 10.5 percent of their initial capacity following formation. This loss in cell capacity is irreversible. Also, if cells are stored even for relatively short periods of time at relatively high temperatures, the cells similarly exhibit irreversible losses in capacity. Because long-term storage and storage at high temperatures following formation are difficult to avoid due to handling by third-party retail sales outlets and consumers, there exists a need for improved NiMH cells that do not exhibit such significant irreversible capacity loss under these somewhat common conditions.
Yet another problem occurs when NiMH cells are deeply discharged below recommended levels, as is the case when a consumer fails to turn off a device powered by a battery pack containing the NiMH cells. Following such deep discharges, the cells experience a significant and irreversible loss in capacity. Such losses may be approximately 10 percent of the cell capacity.